An aerogel is a gel in which the liquid phase has been replaced by air without damaging the solid phase leaving a lattice structure of substantially the same shape as the gel and of only slightly reduced volume. Silica aerogels have a lattice structure formed of amorphous silica (SiO2).
Silica aerogels are extremely light materials, having a specific gravity as low as 0.025 g/cm3, the lowest thermal conductivity of any known solid material, high surface area and high porosity. This makes them very suitable for use in many applications. Very low specific gravity aerogels have been used, for example in aerospace applications as insulating materials on space craft, and particularly on the rover vehicle for the Mars Pathfinder project and as Cerenkov detectors for capturing high velocity cosmic particles, which can easily penetrate the porous material, where they are gradually decelerated to achieve a “soft landing”. The particles trapped in transparent aerogels can even be inspected in situ. They have been used as catalysts and catalyst supports, where their surface area and porosity makes them especially useful, and in insulation and heat storage systems. There are many instances where aerogels could be used or used to a greater extent were their cost not so high. The cost of aerogels, at present, is high both because of their cost of preparation and the cost of the starting materials.
The first commercially produced aerogels were made by a process that comprises adding sulphuric acid to a solution of sodium silicate with the 30 concentrations being controlled to form a gel having 9% silica content. After the gel has aged for several hours to allow it to strengthen, due to syneresis phenomena, it is passed through a roll crusher into a wash tank where water is passed over the gel to remove the sodium sulphate formed in the gel preparation reaction. When the gel has been sufficiently washed, all excess water is removed by draining and the gel is then covered with alcohol to replace the water in the gel with alcohol. After a suitable soaking time the alcohol is drained off and replaced with fresh alcohol. This alcohol washing procedure is repeated several times.
When the water in the gel has been substantially replaced with alcohol, the excess alcohol is drained off and the gel charged to an autoclave, in which it is slowly heated to a temperature above the critical temperature of the alcohol with the pressure being maintained at a level above the critical pressure of the alcohol. When the temperature reaches the desired level, the pressure is reduced to atmospheric and the autoclave is finally evacuated to a level slightly below atmospheric for a short period.
The success of this process is based on one essential step, namely, the heating of a gel system to temperatures and pressures above the critical temperatures and pressures of the liquid phase of the gel, which allows the liquid phase to be removed without destroying the lattice structure of the gel with consequent formation of a dense xerogel rather than a lightweight aerogel.
This process, while it uses fairly cheap reactants, is extremely time consuming because of the need for the washing and multiple solvent exchange steps.
In attempts to quicken the process it was subsequently found that silica aerogels could be produced using tetramethyl orthosilicate (TMOS) as starting material. In this method of operation TMOS is hydrolysed with water in the presence of an acidic or basic catalyst, usually in ethanol. After formation of a silica gel by aging and removal of all remaining water by displacement with alcohol, the gel is dried, as previously, using a super-critical drying technique.
Since TMOS is a dangerous material to use because of its toxicity, its use has now been superseded by tetraethyl and other tetraalkyl orthosilicates that are much safer to use.
Tetraalkyl orthosilicates are however very expensive products and hence the silica aerogels produced from them are similarly expensive and their use is limited to high technology applications. If silica aerogels could be manufactured more cheaply their range of application could be far wider.
The main drawback of the high temperature variants are the severe temperature conditions, causing accelerated aging of the solution-sol-gel samples. It has been proposed to replace the alcohol in the gel with carbon dioxide, which has a very much lower critical temperature than the alcohols generally used, by flushing the vessel and the gel with liquid carbon prior to carrying out the critical drying process.